Speech recognition method and system using compressed speech data

ABSTRACT

A vocoder based voice recognizer recognizes a spoken word using linear prediction coding (LPC) based, vocoder data without completely reconstructing the voice data. The recognizer generates at least one energy estimate per frame of the vocoder data and searches for word boundaries in the vocoder data using the associated energy estimates. If a word is found, the LPC word parameters are extracted from the vocoder data associated with the word and recognition features are calculated from the extracted LPC word parameters. Finally, the recognition features are matched with previously stored recognition features of other words, thereby to recognize the spoken word.

FIELD OF THE INVENTION

The present invention relates to voice recognizers generally and tovoice recognizers which use LPC vocoder data as input.

BACKGROUND OF THE INVENTION

Voice recognizers are well known in the art and are used in manyapplications. For example, voice recognition is used in command andcontrol applications for mobile devices, in computer Dictaphones, inchildren's toys and in car telephones. In all of these systems, thevoice signal is digitized and then parametrized. The parametrized inputsignal is compared to reference parametrized signals whose utterancesare known. The recognized utterance is the utterance associated with thereference signal which best matches the input signal.

Voice recognition systems have found particular use in voice dialingsystems where, when a user says the name of the person he wishes tocall, the voice recognition system recognizes the name from a previouslyprovided reference list and provides the phone number associated withthe recognized name. The telephone then dials the number. The result isthat the user is connected to his destination without having to look forthe dialed number and/or use his hands to dial the number.

Voice dialing is especially important for car mobile telephones wherethe user is typically the driver of the car and thus, must continuallyconcentrate on the road. If the driver wants to call someone, it is muchsafer that the driver speak the name of the person to be called, ratherthan dialing the number himself.

FIG. 1, to which reference is now made, shows the major elements of adigital mobile telephone. Typically, a mobile telephone includes amicrophone 10, a speaker 12, a unit 14 which converts between analog anddigital signals, a vocoder 16 implemented in a digital signal processing(DSP) chip labeled DSP-1, an operating system 18 implemented in amicrocontroller or a central processing unit (CPU), a radio frequencyinterface unit 19 and an antenna 20. On transmit, the microphone 10generates analog voice signals which are digitized by unit 14. Thevocoder 16 compresses the voice samples to reduce the amount of data tobe transmitted, via RF unit 19 and antenna 20, to another mobiletelephone. The antenna 20 of the receiving mobile telephone provides thereceived signal, via RF unit 19, to vocoder 16 which, in turn,decompresses the received signal into voice samples. Unit 14 convertsthe voice samples to an analog signal which speaker 12 projects. Theoperating system 18 controls the operation of the mobile telephone.

For voice dialing systems, the mobile telephone additionally includes avoice recognizer 22, implemented in a separate DSP chip labeled DSP-2,which receives the digitized voice samples as input, parametrizes thevoice signal and matches the parametrized input signal to referencevoice signals. The voice recognizer 22 typically either provides theidentification of the matched signal to the operating system 18 or, if aphone number is associated with the matched signal, the recognizer 22provides the associated phone number.

FIG. 2, to which reference is now made, generally illustrates theoperation of voice recognizer 22. The digitized voice samples areorganized into frames, of a predetermined length such as 5-20 msec, andit is these frames which are provided (step 28) to recognizer 22. Foreach frame, the recognizer 22 first calculates (step 30) the energy ofthe frame.

FIG. 3, to which reference is now also made, illustrates the per frameenergy for the spoken word "RICHARD", as a function of time. The energysignal has two bumps 31 and 33, corresponding with the two syllables ofthe word. Where no word is spoken, as indicated by reference numeral 35,and even between syllables, the energy level is significantly lower.

Thus, the recognizer 22 searches (step 32 of FIG. 2) for the start andend of a word within the energy signal. The start of a word is definedas the point 37 where a significant rise in energy begins after theenergy signal has been low for more than a predetermined length of time.The end of a word is defined as the point 39 where a significant drop inenergy finishes after which the energy signal remains low for more thana predetermined length of time. In FIG. 3, the start point 37 occurs atabout 0.37 sec and endpoint 39 occurs at about 0.85 sec.

If a word is found, as checked in step 34, the voice recognizer 22performs (step 36) a linear prediction coding (LPC) analysis to produceparameters of the spoken word. In step 38, the voice recognizer 22calculates recognition features of the spoken word and, in step 40, thevoice recognizer 22 searches for a match from among recognition featuresof reference words in a reference library. Alternatively, the voicerecognizer 22 stores the recognition features in the reference library,in a process known as "training".

Unfortunately, the voice recognition process is computationallyintensive and, thus, must be implemented in the second DSP chip, DSP-2.This adds significant cost to the mobile telephone.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a voice recognizerwhich operates with compressed voice data, compressed by LPC-based,vocoders, rather than with sampled voice data thereby to reduce theamount of computation which the recognizer must perform. Accordingly,the voice recognition can be implemented in the microcontroller or CPUwhich also implements the operating system. Since the voice recognizerdoes not analyze the voice signal, the microcontroller or CPU can be aof limited processing power and/or one which does not receive the voicesignal.

Moreover, the present invention provides a feature generator which canextract the same type of feature data, for use in recognition, fromdifferent types of LPC based vocoders. Thus, the present inventionperforms the same recognition (e.g. matching and training) operations oncompressed voice data which is compressed by different types of LPCbased vocoders.

There is therefore provided, in accordance with a preferred embodimentof the present invention, a method for recognizing a spoken word usinglinear prediction coding (LPC) based, vocoder data without completelyreconstructing the voice data. The vocoder based recognizer implementsthe method described herein. The method includes the steps of generatingat least one energy estimate per frame of the vocoder data and searchingfor word boundaries in the vocoder data using the associated energyestimates. If a word is found, the LPC word parameters are extractedfrom the vocoder data associated with the word and recognition featuresare calculated from the extracted LPC word parameters. Finally, therecognition features are matched with previously stored recognitionfeatures of other words, thereby to recognize the spoken word.

Additionally, in accordance with a preferred embodiment of the presentinvention, the energy is estimated from residual data found in thevocoder data. This estimation can be performed in many ways. In oneembodiment, the residual data is reconstructed from the vocoder data andthe estimate is formed from the norm of the residual data. In anotherembodiment, a pitch-gain value is extracted from the vocoder data andthis value is used as the energy estimate. In a further embodiment, thepitch-gain values, lag values and remnant data are extracted from thevocoder data. A remnant signal is generated from the remnant data andfrom that, a remnant energy estimate is produced. A non-remnant energyestimate is produced from a non-remnant portion of the residual by usingthe pitch-gain value and a previous energy estimate defined by the lagvalue. Finally, the two energy estimates, remnant and non-remnant, arecombined.

Moreover, in accordance with a preferred embodiment of the presentinvention, the vocoder data can be from any of the following vocoders:Regular Pulse Excitation-Long Term Prediction (RPE-LTP) full and halfrate, Qualcomm Code Excited Linear Prediction (QCELP) 8 and 13 Kbps,Enhanced Variable Rate Codec (EVRC), Low Delay Code Excited LinearPrediction (LD CELP), Vector Sum Excited Linear Prediction (VSELP),Conjugate Structure Algebraic Code Excited Linear Prediction (CS ACELP),Enhanced Full Rate Vocoder and LPC10.

There is also provided, in accordance with a further preferredembodiment of the present invention, a digital cellular telephone whichincludes a mobile telephone operating system, an LPC based vocoder and avocoder based voice recognizer. The recognizer includes a front endprocessor which processes the vocoder data to determine when a word wasspoken and to generate recognition features of the spoken word and arecognizer which at least recognizes the spoken word as one of a set ofreference words.

Further, in accordance with a preferred embodiment of the presentinvention, the front end processor includes an energy estimator, an LPCparameter extractor and a recognition feature generator. The energyestimator uses residual information forming part of the vocoder data toestimate the energy of a voice signal. The LPC parameter extractorextracts the LPC parameters of the vocoder data. The recognition featuregenerator generates the recognition features from the LPC parameters.

Still further, in accordance with a preferred embodiment of the presentinvention, the front end processor is selectably operable with multiplevocoder types.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with theappended drawings in which:

FIG. 1 is a block diagram illustration of a prior art cellular telephonewith voice recognition capabilities;

FIG. 2 is a flow chart illustration of a prior art, LPC-based, voicerecognition method;

FIG. 3 is a graphical illustration of the energy of a spoken word;

FIG. 4 is a schematic illustration of a compressed voice data structure;

FIG. 5 is a block diagram illustration of a cellular telephone with avocoder based voice recognizer, constructed and operative in accordancewith a preferred embodiment of the present invention;

FIG. 6 is a flow chart illustration of a voice recognition method, inaccordance with a preferred embodiment of the present invention;

FIG. 7 is a graphical illustration of the energy of a spoken word asestimated from a residual signal;

FIG. 8 is a graphical illustration of a residual signal, useful inunderstanding the operation of the present invention;

FIG. 9 is a block diagram illustration of a GSM decoder; and

FIG. 10 is a graphical illustration of the energy of a spoken word asestimated from an estimated residual signal.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

There are many types of voice compression algorithms, the most common ofwhich are those based on linear prediction coding (LPC). Applicants haverealized that, since most voice recognition algorithms utilize linearprediction coding analysis in order to parametrize the voice signals,elements of the compressed voice signal can be provided to the voicerecognizer to significantly reduce the amount of analysis which thevoice recognizer must perform. Thus, the present invention is a vocoderbased, voice recognizer to be implemented in the microcontroller or CPUof a cellular mobile telephone, as detailed hereinbelow with respect toFIGS. 5, 6 and 7.

Linear Prediction Analysis:

The following is a short description of the operation of LPC basedvocoder 16. A discussion of speech coding in general, which includes amore complete description of linear prediction coding than that providedhere, can be found in the article "Speech Coding: A Tutorial Review" byAndreas S. Spanias, Proceedings of the IEEE, Vol. 82, No. 10, October1994, pp. 1541-1582.

Vocoder 16 divides the voice signal into a series of frames, each of alength N, typically representing about 20 msec of the voice signal. Oneach frame, vocoder 16 performs linear prediction coding (LPC) analysis.

Linear prediction coding describes a voice signal y(n) as follows:

    y(n)=a.sub.1 y(n-1)+a.sub.2 y(n-2)+ . . . +a.sub.p y(n-p)+ε(n)Equation 1

where the a_(i) are known as the LPC coefficients and ε(n) is known asthe residual signal. Typically, each frame has p LPC coefficients a_(i)and the residual signal ε(n) is of length N. The LPC coefficients andthe residual signal form the parameters of the frame. The vocodertypically further parametrizes the residual signal ε(n) in terms of atleast pitch and gain values. The vocoder can also generate any of themany types of LPC based parameters which are known in the art of LPCvocoders, such as cepstrum coefficients, MEL cepstrum coefficients, linespectral pairs (LSPs), reflection coefficients, log area ratio (LAR)coefficients, etc., all of which are easily calculated from the LPCcoefficients.

The resultant values are then encoded, thereby producing a typical voicecompression frame, such as frame 52 shown in FIG. 4 to which referenceis now made. Voice compression frame 52 includes encoded and/orparametrized versions of the LPC coefficients a_(i) and encoded versionsof the residual signal ε(n).

A Vocoder Based, Voice Recognizer

Reference is now made to FIG. 5 which illustrates a vocoder based, voicerecognizer 50 within a cellular telephone. Since the cellular telephoneis similar to the prior art telephone shown in FIG. 1, similar referencenumerals refer to similar elements. Reference is also made to FIGS. 6and 7 which are useful in understanding the operation of vocoder based,voice recognizer 50.

The cellular telephone of FIG. 5 includes microphone 10, speaker 12,conversion unit 14, vocoder 16, operating system 18, RF interface unit19 and antenna 20. In addition, the cellular telephone of FIG. 5includes vocoder based, voice recognizer 50 which receives the LPC-basedcompressed voice signal, which vocoder 16 produces, as input.

In accordance with a preferred embodiment of the present invention, thevocoder based, voice recognizer 50 is implemented in the device, labeledCPU 51, which also implements the operating system 18. Device 51 can bea CPU, as labeled, or a microcontroller. Since voice recognizer 50 doesnot analyze the voice signal, voice recognizer 50 can be implemented onany type of microcontroller or CPU, including those which have onlylimited processing power and those which do not receive the voicesignal.

FIG. 6 illustrates, in general form, the operations of vocoder based,voice recognizer 50 on a compressed frame such as the frame 52.

As in the prior art, the energy of the frame is determined once theframe, in step 58, has been received. However, in the present invention,the energy is estimated (step 60) from the vocoder data, rather thanfrom the sampled data, and the energy estimation does not involvereconstructing the sampled data.

Applicants have recognized that the residual signal ε(n) can be utilizedto estimate the energy since, as is known in the art, the residualsignal describes the air pressure through the vocal tract while the LPCparameters describe the structure of the vocal tract and are, thus,generally independent of speech volume. As a result, the residual signalis highly correlated to how loudly or quietly a person talks.

In accordance with a preferred embodiment of the present invention, onemethod of estimating the energy is to determine the energy in theresidual signal, per frame, or, if the frames are divided intosubframes, per subframe. Mathematically, this can be written as:##EQU1## where E_(i) is the energy in the ith frame, the residual signalε(n) is reconstructed from the vocoder data and the number M is thenumber of sample points in the frame or subframe.

FIG. 7 illustrates the estimated energy signal produced from thereconstructed residual signals of the voiced word "RICHARD". As can beseen, the estimated energy signal of FIG. 7 is not a replica of theenergy signal of FIG. 3. However, the estimated energy signal is highlycorrelated with the prior art energy signal. The start and end pointsfor the signal of FIG. 7, labeled 62 and 63, respectively, are also atabout 0.37 sec and 0.85 sec, respectively.

Other methods of estimating the energy from the vocoder data areincorporated in the present invention, some of which are describedhereinbelow.

Returning to FIG. 6, the vocoder based, voice recognizer 50 searches(step 64) for word boundaries in the estimated energy signal. Ifdesired, voice recognizer 50 can refine the location of the wordboundaries by using any of the characteristics of the LPC parameters(such as their mean and/or variance) which change sharply at a wordboundary.

If a word is found, as checked by step 66, recognizer 50 extracts (step68) the LPC word parameters from the vocoder data. Step 68 typicallyinvolves decoding the encoded LPC parameters provided in voicecompression frame 52 and converting them to the LPC coefficients.

Recognizer 50 then calculates (step 70) its recognition features fromthe extracted LPC coefficients. These recognition features can be any ofthe many LPC based parameters, such as cepstrum coefficients, MELcepstrum coefficients, line spectral pairs (LSPs), reflectioncoefficients, log area ratio (LAR) coefficients, etc., all of which areeasily calculated from the LPC coefficients. Thus, if the vocoder usesone type of LPC parameter and the recognizer 50 use another type of LPCparameter, recognizer 50 can convert from one to the other eitherdirectly or through the LPC coefficients.

Finally, recognizer 50 utilizes the recognition features produced instep 70 to either recognize the input signal as one of the referencewords in its reference library or to train a new reference word into itslibrary. Since the recognition features produced by recognizer 50 can bethe same as those used in the prior art, this step is equivalent to therecognition/training step 40 of the prior art and thus is so labeled.The book, Fundamentals of Speech Recognition, by Lawrence Rabiner andBiing Hwang Juang, Prentice-Hall, 1993, describes suitable recognizers50 and is incorporated herein by reference.

It will be appreciated that steps 60-70 convert from the vocoder data tothe recognition features needed for the recognition/training step. Thereare many LPC based vocoders, each of which performs somewhat differentoperations on the voice signal. Steps 60-70 can be tailored to each typeof vocoder, in order to produce the same recognition features,regardless of vocoder type. Thus, steps 60-70 form a processing "frontend" to the recognition/training step 40.

The present invention incorporates a vocoder based, voice recognizerwhich has a plurality of front ends and a single recognition/trainingunit. This is particularly useful for those mobile telephones which aresold to operate with multiple types of digital cellular telephonesystems, each of which uses a different type of vocoder. With many frontends, the voice recognizer of the present invention can operate withmany vocoder types.

Energy Estimation Methods for use in Determining the Word Boundaries:

Some simple vocoders, such as the vocoder known as the LPC10 describedin the US Department of Defense standard 1015 V.53, describe theresidual signal ε(n) with just the pitch and gain values. FIG. 8, towhich reference is now made, illustrates an exemplary residual signal,of a voiced signal, which has a series of repeating peaks 70, all ofapproximately the same magnitude. The distance between peaks 70 isdefined as the pitch P and the magnitude of the peaks 70 is defined asthe gain G. A non-voiced signal has a gain value but no pitch value.

Thus, the energy of the residual signal of the frame or subframe can beestimated by the gain value G. In this example, the energy of the frameor subframe is not estimated by reconstructing the residual signal ε(n)but by extracting the gain value G, a parameter of the residual signalε(n), from the compressed voice data.

Other vocoders, such as the vocoders used in Global System for MobileCommunications (GSM), Time Division Multiple Access (TDMA) and CodeDivision Multiple Access (CDMA) digital cellular communication systems,correlate the residual signal of the current frame or subframe with aconcatenated version of the residual signals of previous frames. Thepoint at which the residual signal of the current frame most closelymatches previous residual signals, when multiplied by a pitch gain PG,is known as the LAG value. The vocoders then determine a "remnantsignal" which is the difference between the previous residual signalmultiplied by the pitch gain PG and the current residual signal. Thecurrent residual signal is then characterized by the pitch gain PG, theLAG value and the remnant signal.

For the later type of vocoder, the energy of the current frame orsubframe, i, can be estimated from the remnant signal and from thenon-remnant portion of the residual signal, by: ##EQU2## where E_(rem)is the energy estimate of the remnant signal and E_(LAG) is thenon-remnant energy of the residual, as determined from the energy of theframe or subframe which is ##EQU3## frames or subframes behind thecurrent frame or subframe and the pitch gain. The former can be producedby reconstructing the remnant signal, a relatively simple operation, orby any other method. The symbols .left brkt-top. .right brkt-top. and.left brkt-bot. .right brkt-bot. indicate the "ceiling" and "floor"operations, respectively and the mth root operation need not beperformed.

Energy Estimation for GSM Vocoders

Reference is now briefly made to FIG. 9 which illustrates the decoderportion of a vocoder which forms part of the GSM standard. FIG. 9 issimilar to Fig. 3.4 of the March 1992 version of the I-ETS 300 036specification from the European Telecommunications Standards Institute,found on page 34 thereof. The details of the decoder are provided in theabove-identified specification, which is incorporated herein byreference. For clarity, only the aspects of the decoder necessary forunderstanding the energy and feature calculations of the presentinvention are provided hereinbelow.

FIG. 9 indicates input data with thick lines and internal signals withthin lines. The input data includes the values M_(cr), X_(maxcr),X_(mcr), b_(cr), N_(cr) and LAR_(cr), all of which are defined in theI-ETS specification.

FIG. 9 shows that the decoder includes an RPE decoder 80, a long termpredictor 84, a short term synthesis filter 86, and a de-emphasizer 88.The RPE decoder 80 receives the M_(cr), x_(maxcr) and x_(mcr) signalsand generates a remnant signal e_(r) '. The long term predictor 84 usesthe b_(cr) and N_(cr) signals to generate a residual signal d_(r) ' fromthe remnant signal e_(r) '. The short term synthesis filter 86 generatesthe voice signal from the residual signal d_(r) ' and the short term LPCparameters, transmitted in the form of the LAR_(cr) data.

One energy calculation, similar to that described hereinabove, takes thefirst or second norm of the residual signal d_(r) ', as follows:##EQU4##

Another energy calculation uses the remnant signal e_(r) ' and theinternal data values b_(r) ' and N_(r) ' of the long term predictor 84.Specifically, predictor 84 includes a parameter decoder 90, a delay unit92, a multiplier 94 and a summer 96. Decoder 90 converts the inputvalues b_(cr) and N_(cr) to the internal data values b_(r) ' and N_(r)', where b_(r) ' is a multiplier, similar to the pitch gain PG discussedhereinabove, and N_(r) ' is a delay amount, similar to the value LAGdiscussed hereinabove. Long term predictor 82 adds the signal d_(r) " tothe remnant signal e_(r) ', where the signal d_(r) " is the previousresidual signal d_(r) '(I-N_(r) '), as delayed by delay unit 92,multiplied by an amount b_(r) ' via multiplier 94.

The energy can be estimated using Equation 3, where N_(r) ' and b_(r) 'replace the LAG and PG values and FL is set to 40. Furthermore, theenergy estimate of the remnant, E_(rem), is calculated by: ##EQU5##

FIG. 10, to which reference is now briefly made, shows the estimatedenergy using the above calculation. The start and stop word boundaries,labeled 98 and 99, respectively occur at the same locations as in theprior art.

Another method of estimating the energy from the extracted parametersalso uses N_(r) ' and b_(r) ' as above, with FL set to 40, and estimatesthe energy estimate of the remnant, E_(rem), as:

    E.sub.rem =|X.sub.maxcr |.sup.m          Equation 6

Returning to FIG. 9, the LPC word parameters are extracted from thetransmitted data within the short term synthesis filter 86 whichincludes an LAR decoder 100, an interpolator 102, a reflectioncoefficients determining unit 104 and a filter 106. Together, units 100,102 and 104 convert the received LAR_(cr) data to the reflectioncoefficients r_(r) ', where the latter are easily transformed into LPCcoefficients.

As mentioned hereinabove with respect to FIG. 6, once the LPCcoefficients are extracted, they are transformed (step 70) into therecognition features which the recognizer/training step requires.

It will be appreciated by those skilled in the art that, while a fullexplanation has been provided for the vocoder of the GSM digitalcellular communication system, the present invention is applicable toall types of digital cellular communication systems and to all types ofLPC-based vocoders. For each type of vocoder, the type of informationstored in the compressed voice data must be analyzed to determine how toutilize it for the energy and feature calculations. The compressed voicedata is described in detail in the standard defining each vocoder.

The following table lists some currently available cellularcommunication systems, the vocoders they work with and the standardsdefining the vocoders and/or the systems.

    ______________________________________                                        Digital Cellular                                                              Communication System                                                                       LPC-based Vocoder                                                                           Standard                                           ______________________________________                                        GSM          RPE-LTP full rate                                                                           I-ETS 300 036 6.1                                               RPE-LTP half rate                                                                           I-ETS 300 581-2                                                               ver. 4                                             CDMA         QCELP 8 Kbps, IS 96 A                                                         13 Kbps                                                                       EVRC          IS 127                                                          LD CELP       ITU G.728                                          TDMA         VSELP         IS 54 B                                            PHS, PCS     CS ACELP      ITU G.729                                          PCS-TDMA     Enhanced Full Rate                                                                          IS 641                                                          Vocoder                                                          PDC (in Japan)                                                                             VSELP         RCR STD 27                                         ______________________________________                                    

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed herein above. Rather the scope of the invention is defined bythe claims that follow:

We claim:
 1. A method for recognizing a spoken word, the methodcomprising the steps of:receiving compressed speech data that has beencompressed using linear prediction coding (LPC) techniques; extractingat least one set of LPC parameters from said compressed speech datawithout completely decompressing said compressed speech data;calculating at least one recognition feature from said at least one setof LPC parameters; and utilizing said at least one recognition featureand at least one previously stored recognition feature to recognize thespoken word.
 2. A method according to claim 1 and wherein saidcompressed speech data is of the type produced by any of the followingvocoders: Regular Pulse Excitation-Long Term Prediction (RPE-LTP) fulland half rate, Qualcomm Code Excited Linear Prediction (QCELP) 8 and 13Kbps, Enhanced Variable Rate Codec (EVRC), Low Delay Code Excited LinearPrediction (LD CELP), Vector Sum Excited Linear Prediction (VSELP),Conjugate Structure Algebraic Code Excited Linear Prediction (CS ACELP),Enhanced Full Rate Vocoder and 10 Linear Prediction Coefficients(LPC10).
 3. A method according to claim 1, the method further comprisingthe steps of:generating at least one energy estimate for at least aportion of said compressed speech data without completely decompressingsaid compressed speech data; and searching for at least two wordboundaries in said compressed speech data in accordance with said atleast one energy estimate, and said at least one set of LPC parametersis from an area between said at least two boundaries.
 4. A methodaccording to claim 3, wherein said portion includes at least one frame.5. A method according to claim 4 and wherein said step of generatingcomprises the step of estimating the energy from residual data found insaid compressed speech data.
 6. A method according to claim 5 andwherein said step of estimating comprises the steps of reconstructingresidual data from said compressed speech data and generating the normof said residual data.
 7. A method according to claim 5 and wherein saidstep of estimating comprises the steps of extracting a pitch-gain valuefrom said compressed speech data and using said extracted pitch-gainvalue as said energy estimate.
 8. A method according to claim 5 andwherein said step of generating comprises the steps of:extractingpitch-gain values, lag values and remnant data from said compressedspeech data; reconstructing a remnant signal from said remnant data;generating an energy estimate of said remnant signal; generating anenergy estimate of a non-remnant portion of said residual by using saidpitch-gain value and a previous energy estimate defined by said lagvalue; and combining said remnant and non-remnant energy estimates.
 9. Amethod for preparing to recognize a spoken word, the method comprisingthe steps of:receiving compressed speech data that has been compressedusing linear prediction coding (LPC) techniques; extracting at least oneset of LPC parameters from said compressed speech data withoutcompletely decompressing said compressed speech data; and calculating atleast one recognition feature from said at least one set of LPCparameters.
 10. A method according to claim 9 and wherein saidcompressed speech data is of the type produced by any of the followingvocoders: RPE-LTP full and half rate, QCELP 8 and 13 Kbps, EVRC, LDCELP, VSELP, CS ACELP, Enhanced Full Rate Vocoder and LPC
 10. 11. Amethod according to claim 9, the method further comprising the stepsof:generating at least one energy estimate for at least a portion ofsaid compressed speech data without completely decompressing saidcompressed speech data; and searching for at least two word boundariesin said compressed speech data in accordance with said at least oneenergy estimate, and said at least one set of LPC parameters is from anarea between said at least two boundaries.
 12. A method according toclaim 11, wherein said portion includes at least one frame.
 13. Adigital cellular telephone comprising:a mobile telephone operatingsystem; a vocoder which compresses a voice signal using at least linearprediction coding (LPC) thereby to produce compressed speech data; and aspeech recognizer comprising:a front end processor, operating on saidcompressed speech data without completely decompressing said compressedspeech data, which determines when a word was spoken and generatesrecognition features of said spoken word; and a recognition unit whichat least recognizes said spoken word as one of a set of reference words.14. A digital cellular telephone according to claim 13 and wherein saidfront end processor includes:an energy estimator which uses residualinformation forming part of said compressed speech data to estimate theenergy of a voice signal; an LPC parameter extractor which extracts theLPC parameters of said compressed speech data; and a recognition featuregenerator which generates said recognition features from said LPCparameters.
 15. A cellular telephone according to claim 13 and whereinsaid vocoder is any of the following vocoders: Regular PulseExcitation-Long Term Prediction (RPE-LTP) full and half rate, QualcommCode Excited Linear Prediction (QCELP) 8 and 13 Kbps, Enhanced VariableRate Codec (EVRC), Low Delay Code Excited Linear Prediction (LD CELP),Vector Sum Excited Linear Prediction (VSELP), Conjugate StructureAlgebraic Code Excited Linear Prediction (CS ACELP), Enhanced Full RateVocoder and 10 Linear Prediction Coefficients (LPC10).
 16. A digitalcellular telephone according to claim 13, wherein said recognition unitfurther comprises a training unit for storing recognition features ofsaid set of reference words.
 17. A speech recognizer operable withcompressed speech data which has been compressed using linear predictioncoding (LPC) techniques by a vocoder, the speech recognizer comprising:afront end processor which processes said compressed speech data withoutcompletely decompressing said compressed speech data to determine when aword was spoken and to generate recognition features of said spokenword; and a recognition unit which at least recognizes said spoken wordas one of a set of reference words.
 18. A speech recognizer according toclaim 17 and wherein said front end processor comprises:an energyestimator which uses residual information forming part of saidcompressed speech data to estimate the energy of a voice signal; an LPCparameter extractor which extracts the LPC parameters of said compressedspeech data; and a recognition feature generator which generates saidrecognition features from said LPC parameters.
 19. A speech recognizeraccording to claim 18 and wherein said energy estimator comprises aresidual energy estimator which estimates the energy from residual datafound in said compressed speech data.
 20. A speech recognizer accordingto claim 19 and wherein said residual energy estimator comprises aresidual reconstructor which reconstructs residual data from saidcompressed speech data and a norm generator which generates the norm ofsaid residual data thereby to produce said energy estimate.
 21. A speechrecognizer according to claim 19 and wherein said residual energyestimator comprises an extractor which extracts a pitch-gain value fromsaid compressed speech data thereby to produce said energy estimate. 22.A speech recognizer according to claim 19 and wherein said residualenergy estimator comprises:an extractor which extracts pitch-gainvalues, lag values and remnant data from said compressed speech data; areconstructor which reconstructs a remnant signal from said remnantdata; a remnant energy estimator which generates an energy estimate ofsaid remnant signal; a non-remnant energy estimator which generates anenergy estimate of a non-remnant portion of said residual by using saidpitch-gain value and a previous energy estimate defined by said lagvalue; and a combiner which combines said remnant and non-remnant energyestimates thereby to produce said energy estimate.
 23. A speechrecognizer according to claim 17 and wherein said vocoder is any of thefollowing vocoders: Regular Pulse Excitation-Long Term Prediction(RPE-LTP) full and half rate, Qualcomm Code Excited Linear Prediction(QCELP) 8 and 13 Kbps, Enhanced Variable Rate Codec (EVRC), Low DelayCode Excited Linear Prediction (LD CELP), Vector Sum Excited LinearPrediction (VSELP), Conjugate Structure Algebraic Code Excited LinearPrediction (CS ACELP), Enhanced Full Rate Vocoder and 10 LinearPrediction Coefficients (LPC10).
 24. A speech recognizer according toclaim 17, wherein said recognition unit further comprises a trainingunit for storing recognition features of said set of reference words.25. A digital cellular telephone comprising:a mobile telephone operatingsystem; a plurality of vocoders which compress a voice signal using atleast linear prediction coding (LPC) thereby to produce compressedspeech data, each vocoder operable with one of a corresponding pluralityof vocoder types; and a speech recognizer comprising:a correspondingplurality of front end processors, one for each of said vocoder types,each said processor operable on said compressed speech data withoutcompletely decompressing said compressed speech data, which determinewhen a word was spoken and generate recognition features of said spokenword; and a recognition unit which at least recognizes said spoken wordas one of a set of reference words.
 26. A digital cellular telephoneaccording to claim 25, wherein said recognition unit further comprises atraining unit for storing recognition features of said set of referencewords.